Detect closed network connection

Question

I've written a number of small programs that communicate via TCP. I'm having endless issues with the system hanging because one program has closed its network connection, and the other end-point somehow fails to notice that it's now disconnected.

I was expecting doing I/O on a TCP connection that has been closed to throw some kind of I/O exception, but instead the program seems to just hang, waiting forever for the other end-point to reply. Obviously if the connection is closed, that reply is never coming. (It doesn't even seem to time out if you leave it for, say, twenty minutes.)

Is there some way I can force the remote end to "see" that I've closed the network connection?

Update: Here is some code...

public sealed class Client
{
  public void Connect(IPAddress target)
  {
    var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
    socket.Connect(ipAddress, 1177);
    _stream = new NetworkStream(socket);
  }

  public void Disconnect()
  {
    _stream.Close();
  }
}

public sealed class Server
{
  public void Listen()
  {
    var listener = new TcpListener(IPAddress.Any, 1177);
    listener.Start();
    var socket = listener.AcceptSocket();
    _stream = new NetworkStream(socket);
    ...
  }

  public void Disconnect()
  {
    socket.Shutdown(SocketShutdown.Both);
    socket.Disconnect(false);
  }
}

In TCP a process normally can't hang forever... TCP sends `alive?` signals to check if the other node is still active. — Willem Van Onsem, Sep 17 '13 at 09:49
Have you considered `TcpClient.LingerState`? How do you use TCP? Socket Programming or some dedicated library? — Willem Van Onsem, Sep 17 '13 at 09:52
There is a loop somewhere where you are receiving data inside the program that seems to hang. Post that code. — Jon, Sep 17 '13 at 10:09

C.Evenhuis · Accepted Answer · 2013-09-17T10:33:22.253

2

When an application closes a socket the right way, it sends a message containing 0 bytes. In some cases you may get a SocketException indicating something went wrong. In a third situation, the remote party is no longer connected (for instance by unplugging the network cable) without any communication between the two parties.

If that last thing happens, you'll have to write data to the socket in order to detect that you can no longer reach the remote party. This is why keep-alive mechanisms were invented - they check every so often whether they can still communicate with the other side.

Seeing the code you posted now: when using NetworkStream the Read operation on it would return a value of 0 (bytes) to indicate that the client has closed the connection.

The documentation is mentions both

"If no data is available for reading, the Read method returns 0."

and

"If the remote host shuts down the connection, and all available data has been received, the Read method completes immediately and return zero bytes."

in the same paragraph. In reality NetworkStream blocks if no data is available for reading while the connection is open.

edited Sep 17 '13 at 10:33

answered Sep 17 '13 at 10:09

C.Evenhuis

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You mean `Read()` returns zero bytes rather than throwing an exception? Oh dear... – MathematicalOrchid Sep 17 '13 at 10:26
@MathematicalOrchid exactly – C.Evenhuis Sep 17 '13 at 10:29
So presumably `ReadByte()` is also going to return `-1` rather than throw an actual exception too... – MathematicalOrchid Sep 17 '13 at 10:33
@MathematicalOrchid they had to choose something other than exceptions, as a correctly shut down socket is not an exceptional case; and the default `ReadByte` implementation calls `Read` under the hood. – C.Evenhuis Sep 17 '13 at 10:35
I was hoping to not have to litter my code with dozens of identical checks for this. (That's kind of why exception handling was invented...) Oh well, I guess it's not your fault that they designed it this way. :-( – MathematicalOrchid Sep 17 '13 at 10:42

score 1 · Answer 2 · edited May 23 '17 at 10:25

1

Hi MathematicalOrchid,

You might find what you are looking for here:

http://blog.stephencleary.com/2009/05/detection-of-half-open-dropped.html

There is some great information there when it comes to working with TCP sockets and detecting half open connections.

You can also refer to this post which seems to have the same solution:

TcpClient communication with server to keep alive connection in c#?

-Dave

edited May 23 '17 at 10:25

Community

1
1

answered Sep 17 '13 at 10:21

TheDaveJay

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This is interesting, but it seems to be talking about when a process dies, or a network link goes down. I'm trying to _explicitly_ close the connection at one end, which ought to notify the other end, you would think... – MathematicalOrchid Sep 17 '13 at 10:40

Kami · Answer 3 · 2013-09-17T10:30:55.027

1

You are opening the socket, and assigning it to the stream. At the end of the process, you close the network stream, but not the socket.

For NetworkStream.Close() to close the underlying socket it must have the ownership parameters set to true in the constructor - See MSDN Docs at http://msdn.microsoft.com/en-us/library/te7e60bx.aspx.

This may result in the connection hanging as the underlying socket was not correctly closed.

Change

_stream = new NetworkStream(socket);

To

_stream = new NetworkStream(socket, true);

On a side note, if you do not require a maximum performance for your small app you should try using TCPClient instead - http://msdn.microsoft.com/en-us/library/system.net.sockets.tcpclient%28v=vs.100%29.aspx. This is a wrapper around socket and it provides connection state checking facilities.

edited Sep 17 '13 at 10:30

answered Sep 17 '13 at 10:25

Kami

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Is a similar step necessary if I wrap the stream in a `StreamReader` or `StreamWriter` object? – MathematicalOrchid Sep 17 '13 at 13:01
@MathematicalOrchid `StreamReader/StreamWriter` do not have socket constructors, so you would have to wrap a them around a `NetworkStream`. This would still result in the above scenario. Also, `StreamReader` will block until it receives data - If there is no data (connection lost or something) then you may get a timeout or indefinite hang. Your best option might to be use/implement some kind of keep alive. – Kami Sep 17 '13 at 13:52

Detect closed network connection

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